1. Field of the Invention
The present invention relates to a motor driving method, a program therefor, and a motor driving apparatus, which are applied to a driving circuit of a vibration wave motor for relatively moving a driven member which is in contact with an elastic member (i.e., vibration member) having a vibration wave generated therein by a frictional force.
2. Description of the Related Art
In recent years, a vibration wave (i.e., vibration type) motor of a nonelectromagnetic driving system called a vibration wave motor or a piezoelectric motor has been developed, and put into practical use by the applicants of the present invention. The vibration wave motor is configured to apply an alternating voltage to an electro-mechanical energy conversion element such as a piezoelectric element or an electrostrictive element thereby generating a high-frequency vibration therein, and to take out vibration energy as continuous mechanical motion. An operation principle of the vibration wave motor is described in many patent applications such as in Japanese Patent Application Laid-Open No. H03-289375, and thus description thereof will be omitted.
FIG. 12 is a perspective view showing an appearance of a vibration member of a vibration wave actuator of a linear driving type according to a conventional example.
Referring to FIG. 12, a vibration member 101 includes an elastic member 104, a piezoelectric element (electromechanical energy conversion element) 105, and two projection portions 106. As a technology regarding the vibration wave actuator is well-known in, e.g., Japanese Patent Application Laid-Open No. 2004-320846, detailed description thereof will be omitted. The elastic member 104 is made of a metallic material and formed into a rectangular plate shape. The piezoelectric element 105 is bonded to a back surface of the elastic member 104, and the projection portions 106 are disposed in a front surface of the elastic member 104.
The vibration member 101 can excite vibrations of two bending vibration modes, and generates elliptic motion in tips of the projection portions 106 by combining the vibrations of the two bending vibration modes. In other words, the projection portions 106 are designed such that the tips are brought into contact with a driven member (i.e., slider) as described below to move the driven member by elliptic motion generated in the tips.
FIGS. 13A and 13B are views showing vibrations of the two bending vibration modes of the vibration member 101. FIG. 13A shows a vibration of one bending vibration mode, and FIG. 13B shows a vibration of the other bending vibration mode.
The vibration mode of FIG. 13A represents one (referred to as “A mode” hereinafter) of the two bending vibration modes. A vibration of the A mode is a secondary bending vibration in a longitudinal direction (i.e., direction of the arrow X) of the rectangular vibration member 101 (or elastic member 104), and has three parallel nodes in a short direction (i.e., direction of the arrow Y). The projection portions 106 are arranged near positions which become nodes in vibration of the A mode, and reciprocated in the direction of the arrow X by the vibration of the A mode. With this arrangement of the projection portions 106, it is possible to displace the projection portions 106 most greatly in the direction of the arrow X.
The vibration mode of FIG. 13B represents one (referred to as “B mode” hereinafter) of the two bending vibration modes. A vibration of the B mode is a primary bending vibration in a short direction (i.e., direction of the arrow Y) of the rectangular vibration member 101 (or elastic member 104), and has two parallel nodes in a longitudinal direction (i.e., direction of the arrow X). The nodes in the vibration of the A mode and the nodes in the vibration of the B mode are roughly orthogonal to each other in an XY plane. The projection portions 106 are arranged near positions where an antinode occurs in vibration of the B mode, and reciprocated in a direction of an arrow Z by the vibration of the B mode. With this arrangement of the projection portions 106, it is possible to displace the projection portions 106 most greatly in the direction of the arrow Z.
In other words, by setting the nodes in the vibrations of the A and B modes roughly orthogonal to each other as described above, the positions of the nodes in the vibration of the A mode can be matched with the positions of the antinode in the vibration of the B mode. By arranging the projection portions 106 in those positions, vibration displacements of the projection portions 106 can be made greatest, enabling acquisition of a high output. By greatly displacing the projection portions 106 in the directions of the arrows X and Z, it is possible to apply a large driving force to the driven member brought into contact with the projection portions 106.
FIG. 14 is a perspective view showing an appearance of a vibration wave actuator which uses the vibration member of FIG. 12.
Referring to FIG. 14, the vibration wave actuator moves a slider 107 by the vibration member 101 constituted of the elastic member 104, the piezoelectric element 105, and the projection portions 106. The vibration wave actuator can generate elliptic motion in the tips of the projection portions 106 by generating vibrations of the A and B modes with a predetermined phase difference. The slider 107 serving as a driven member is pressed into contact with the tips of the projection portions 106. The slider 107 can be moved in a direction of the arrow L by the elliptic motion of the projection portions 106.
By arranging the two projection portions 106 symmetrically with respect to an XZ plane or a YZ plane passing through a center of the elastic member 104, the vibration member 101 can unevenly receive a reaction force received from the slider 107 on the projection portions 106. Because of a stable relative positional relation between the slider 107 and the projection portions 106, it is possible to stabilize an output of the vibration member 101 without any influence of a fluctuation or the like in environment or load.
In the vibration wave motor, as disclosed in Japanese Patent Application Laid-Open No. H01-085587 or JP 3382454 B, when speed control is executed, a method of gradually reducing a driving frequency from a high side and executing pulse width control or phase difference control by a driving frequency when a certain speed is reached is employed.
FIG. 15 shows a relation between an AB phase difference and a motor position described below with respect to time in control of executing frequency sweeping in a vibration wave motor of another conventional example. The frequency sweeping refers to an operation of gradually changing a frequency from high to low.
In FIG. 15, an abscissa indicates time, while an ordinate indicates a phase difference (referred to as “AB phase difference” hereinafter) of two-phase driving voltages (i.e., driving voltages of A and B phases) applied to the piezoelectric element of the vibration wave motor. Characteristics shown in the drawing indicate a relation between the AB phase difference and the motor position in a case where when frequency sweeping is carried out and a driving frequency reaches a target speed, the driving frequency is fixed at this frequency, and the apparatus is operated while changing a target position of the slider with the AB phase difference.
After execution of the frequency sweeping by fixing the AB phase difference, it is possible to drive the vibration wave motor of a driving frequency at which control characteristics of AB phase difference control are stabilized by using an algorithm for deciding a driving frequency at which the driven member is set to a certain speed. A numerical value of the ordinate of the right side of the characteristics shown in FIG. 15 is a count value of an encoder for detecting a slider moving amount.
However, as the frequency sweeping is an operation which relatively requires time, it takes time to start the vibration wave motor.